1. Field of the Invention
The present invention relates to an operation mode transfer system and an operation mode transfer method that transfer an operative or inoperative mode of a device connected to a host computer according to the USB (Universal Serial Bus Specification Revision 1.0, Jan. 15, 1996) standard.
2. Description of Related Art
FIG. 9 is a circuit diagram showing a relationship between a host computer and a device according to the USB standard. In FIG. 9, the reference numeral 1 designates a device like a mouse, a keyboard, a data tablet, or a game pad; 2 designates a pullup power supply; 3 designates a USB standard pullup resistor for pulling up a signal line D.sup.- according to the same standard; 4 designates a connector for connecting the device 1 with a host computer 5 such as a personal computer; 6 designates an input/output portion of the host computer 5 for receiving and transmitting data from and to the device 1 through the signal lines D.sup.- and D.sup.+ ; 7 designates a USB standard pulldown resistor for pulling down the signal line D.sup.- ; and 8 designates a USB standard pulldown resistor for pulling down the signal line D.sup.+.
Next, the operation of the conventional system will be described.
First, according to the USB standard communication scheme, the host computer 5 and device 1 are interconnected through four lines (the signal lines D.sup.- and D.sup.+, 5V power supply V.sub.cc, and ground wire GND) as shown in FIG. 9.
With this arrangement, the device 1 transmits data to the host computer 5 by applying a differential signal between the signal lines D.sup.- and D.sup.+. Specifically, the device 1 applies an L level signal to the signal line D.sup.- when applying an H level signal to the signal line D.sup.+, and vice versa. Thus, the input/output portion 6 of the host computer 5 receives the data from the device 1, and a CPU of the host computer 5 not shown in FIG. 9 analyzes the contents of the data.
However, the system with such a configuration has no operation mode transfer system that notifies the host computer 5 about the operative or inoperative mode of the device 1 while the device 1 is connected to the host computer 5. Accordingly, the host computer 5 considers that the device 1 is in the operative mode as long as they are physically interconnected, and hence the CPU of the host computer 5 must periodically check the receiving state of the input/output portion 6.
Incidentally, while the device 1 is logically connected with the host computer 5, the potential V.sub.M of the signal line D.sup.- is kept higher than a set value V.sub.TH1 as shown in FIG. 10A because it is pulled up by the pullup power supply 2 through the pullup resistor 3 as shown in FIG. 9. On the contrary, while the device 1 is logically disconnected from the host computer 5, the potential V.sub.M of the signal line D.sup.- is kept lower than a set value V.sub.TH2 as shown in FIG. 10B because it is not pulled up.
Thus, the host computer 5 can make a decision whether the device 1 is logically connected or not by comparing the potential V.sub.M of the signal line D.sup.- with the set values V.sub.TH1 and V.sub.TH2.
With the foregoing configuration, although the host computer 5 can decide whether the device 1 is physically connected or not, it cannot make a decision whether the device 1 is operative or not because the conventional system has no operation mode transfer system. Accordingly, even if the device 1 is inoperative in actuality, the host computer 5 must continue a communication processing like that carried out while the device 1 is operative as long as the device 1 is physically connected. This presents a problem in that the processing load of the host computer 5 cannot be reduced even if the device 1 is inoperative mode.
In addition, even if the device 1 is not actually operative, a current flows from the pullup power supply 2 to the ground through the pullup resistor 3 and pulldown resistor 7 as long as the device 1 is physically connected with the host computer 5. This arises another problem in that a current consumption cannot be reduced.